This invention relates to a porous fluorine resin membrane and a process for preparing the same.
As porous fluorine type resin membrane having excellent chemical resistance and thermal resistance, there have been known the following ones. Japanese Unexamined Patent Publications Nos. 136354/1975, 158465/1979 and 147030/1984 are known to disclose porous membranes made of an ethylene-tetrafluoroethylene copolymer and processes for preparing the same. The Japanese Unexamined Patent Publication No. 136354/1975 discloses a process in which fine powder of an ethylene-tetrafluoroethylene copolymer is subjected to styrene polymerization by preparing a mixed solution of a styrene monomer and a slurry to form a membrane, from which a styrene polymer is eluted to form a porous membrane. The porous membrane obtained by this process, however, has a pore size of as large as 10.mu. and, moreover, has very poor permeability, thereby being unsuitable for a microfilter. The Japanese Unexamined Patent Publication No. 158465/1979 discloses a process in which a film of an ethylene-tetrafluoroethylene copolymer is subjected to charging particle irradiation, followed by etching with an aqueous solution of sodium hydroxide to form a porous membrane. However, the porous membrane thus obtained, having no three dimensional network structure, is not only poor in its performance but also inferior in the mechanical properties, thereby making it impossible to obtain a uniform, hollow fibrous porous membrane. Moreover, there is a problem that the process is not suited for a mass production since it uses a nuclear reactor. The Japanese Unexamined Patent Publication No. 147030/1984 discloses a process in which a film of an ethylene-tetrafluoroethylene copolymer is coated with a resist to form a resist pattern having holes, and thereafter a thru-hole corresponding to the resist pattern is formed by sputter etching treatment to obtain a porous membrane. The porous membrane obtained by this process, however, which also has no three dimensional network structure, is also not only poor in its performance but also inferior in the mechanical properties, thereby making it difficult to obtain a uniform, hollow fibrous porous membrane. Moreover, there is a problem in the productivity since the process requires the sputter etching over a long period of time.
As a process having solved the above problems, Japanese Unexamined Patent Publications Nos. 79011/1980 159128/1981, 28139/1982, 93798/1983, 179297/1983, etc. are known to disclose a process in which an ethylene-tetrafluoroethylene copolymer, fine powder of silica and dioctylphthalate are mixed and melt-molded, followed by extraction of the fine powder of silica and dioctylphthalate from the molded product to form a porous membrane. However, the porous membrane obtained by this process has not sufficient uniformity in the pore structure, and contains a number of voids which are extraordinarily large as compared with its mean pore size. Accordingly, although this porous membrane is a membrane having a three dimensional network structure, the number of fibers constituting the network is small in the thicknesswise direction of the membrane, and the performance to eliminate fine particles is inferior when it is used as a microfilter. Moreover, this porous membrane generates pinholes (abnormal coarse communicated holes) very frequently, and has a problem of inconsistency in the quality (great variance in performance) of the membrane and inferiority in the productivity (yield of an article of good quality).
Thus, as mentioned above, none of the conventionally available porous membranes made of an ethylene - tetrafluoroethylene copolymer and processes for preparing the same have been satisfactory.
As a porous membrane made of polychlorotrifluoroethylene, there is a diaphragm membrane for use in electrolysis as disclosed in Japanese Unexamined Patent Publications Nos. 34081/1972 and 25065/1973. However, these membranes, which are used as diaphragm membranes for electrolysis, have extremely low permeability, and are not suited for a porous membrane for use in a microfilter. Accordingly, none of the conventionally available porous membranes made of polychlorotrifluoroethylene and processes for preparing the same have been satisfactory.
Further, nothing has been known conventionally as to membranes comprising an ethylene-chlorotrifluoroethylene copolymer and processes for preparing the same.